NATO SAID yesterday that it has hit one in five tanks and pieces of heavy armour in Kosovo and that the Yugoslav forces there are largely pinned down, isolated, and suffering from the effects of dwindling supplies and low morale.

However, in its most detailed account so far of the effects of the air campaign, the alliance conceded that it was still some way from achieving its military objectives and had not stopped ethnic cleansing in Kosovo.

A detailed briefing by General Walter Jertz gave an upbeat assessment of the success of recent air strikes. "We have pretty much pinned them down, pretty much cut them off and are about to begin to take them out," General Jertz said.

The alliance has drawn up new contingency plans for entering Kosovo with ground troops, but yesterday's briefing was to demonstrate the results of nearly 17,000 air sorties.

The general said that, except for the Danube bridge in Belgrade, all but two of the 31 bridges across the river had been destroyed. The two major rail routes into Kosovo have been closed, and the two biggest roads linking Serbia and Kosovo made impassable.

All oil-refining capacity has been destroyed as have 70 per cent of military stocks and one- third of fuel storage capacity.

The general said that eight important command posts had been destroyed and that Nato had struck an estimated 50 per cent of ammunition storage facilities. Three hundred pieces of military equipment had been attacked, representing damage to about 20 per cent of the heavy forces in Kosovo.

General Jertz claimed that Serb units were able to move only "furtively", and that their morale had been hit by harsh conditions, poor food, low pay, lack of sleep and fear of attack. But he added that "ethnic cleansing is still going on".